Loud speaker



June 11, 1935. A. A. THOMAS 2,004,735

' LOUD SPEAKER Filed July 27, 1928 2 Sheets-Sheet l June 11, 1935. THQMAS 2,004,735

I LOUD SPEAKER led July 27, 1923 2 Sheets-Sheet 2 -%WMW Patented June 11, 1935 I UNITED sTATEs;

2,004,735 LOUD SPEAKER Adolph 'A. Thomas, New York, N. Y., assignor to Radio Corporation of America, New York, N. Y., a corporation of Delaware Application, July 27, 1928, Serial No. 295,638

11 Claims.

My invention relates to loudspeakers of the dynamic type, in which the diaphragm is operated by a speech coil movable in a magnetic airgap. In prior loudspeakers of this kind,

there is a central pole piece which carries a fixed magnetizing coil, and a vibratory speech coil surrounds this pole piece. In other words, the two coils are mounted in axial alignment on the same magnetic core and are thereby closely coupled magnetically. Consequently, the variable magnetic flux produced by the current impulses in tha sp eech coil reacts with maximum transformer effect on the field coiland causes a magnetic disturbance with resultantqdistortion int-the acoustic reproduction.

One of the objects of my invention is to over,- comethe foregoing difficulty by eliminating or minimizing, the inductive effect, between the field coil and the speechcoil of a dynamic speaker. For this purpose I provide a magnetic field structure in which the magnetizing coil is at right angles to thespeech coil, whereby the inductive action ofthe latter on the field Winding is reduced to a negligible quantity. If desired, a separate neutralizing 'coil'connected in the talking circuitmay be mounted on the central pole piece over which the speech "coil' vibrates. This neutraliz'ing coil produces a variable magnetic flux adapted to opposeand neutralize as much as possible the electromagnetic reaction of the speech coil on the normal field flux passing throughthe pole piece.

. Another feature of my new loudspeaker comprises a speech coil with two windings adapted to accentuate, respectively, the upper and the lower tonal registers, so that-the instrument reproduces the entire musical scale with substantially uniform efliciency. The speech coil'is preier'ably mounted directly'on the diaphragm and is held in operative position by 'a light no n-znagnetic'shell of novel construction. My invention also comprises a loudspeaking cone diaphragm of novel construction and improved operation. The central or main body of the cone is surrounded by a reversely bent flange which 'actslike an elastic ring to support the main body of the diaphragm for vibratory move-- ment. Thisdiaphragm may be a compound structure; having a central conical section in reverse relation to the surrounding'conical section and preferably of difierent material. The central section responds best to the higher frequencies, and the outer section to the lower frequencies, lsothat the resultant reproduction covers the entire musical-scale with practically'equal fidelity.

ersalong the same line. To make the instrument solid, or ,itrhayhave openings: l9 fox" the emis- (Cl. 17s--115.5)

Various other [practical features anda'dvan tages of. my. invention will become apparent from a detailed description of the: embodimentsshown in the accompanying drawings, in Which-- Fig. 1 shows a vertical cross-section of aloudspeaker constructed inacco rdance with my i-nvention; v j r I Fig. 2 is a detachedface view of the fieldmagnet frame embodied in Fig. .1; d v d Fig. 3 is a detached perspective view'of the m cylindrical mounting for, the vibratory speech 6 1;

Fig.4 indicates diagrammatically the circuit connections of the coils shown in Fig. 1; 1

Fig.6 illustrates a detached face view of a modified construction ofmagnetic field-frame in which the body of the magnet is laminated;

Fig. 6 is a plan or edge View of Fig.5; Fig. 7 shows a rear View on line 'l-e-l of Fig. 1; Fig. 8 is a fragmentary view in section on the broken line 88.of Fig. 7; J Fig. e illustrates a modification, partly in vertical section; and a l Fig. 10 is a diagram ofcircuit connectionsifor the coils embodied in theconstruction of Fig". 9. Referringto Fig.1, there is a basetlll provided with a hollow post H! which carries a cylindrical housing l3. The parts Ill, I2 "and l3 may be formed as a single castingof non-magnetic ma.- terial, such as aluminum, brass, bakeliteand othstable, the fiaring base l0 should preferably be weighted, or it can be made of heavy metal: sepa-, rate from the post l2 and housing l3. "The base I8 and post I2 may be cast in onepiece of heavy metal, like brass, and the housing I3,may be a -15, or in any other practical way. An integral annular shoulder [don casing l4 forins a 'sto'p to limit the axial movement/of thecas'irig' into the supporting housing I3. The cas'ing' 'Hfisprovided with a conical extension l'lywhich terminates in a ring is having a face orcdnt'actdge 88' arranged at an angle with respect to theaxis of the casing. The conical extension l1 may; be

sion of sound. Thering I8 is adapted to receive theedge of a cone diaphragm 20', and ajconical ring 21 fits over the edge of the diaphragm t'o secure the same against the conical annular 'shoul der l8 of the flaring extension 11. Screws Hb'r Thediaphragm has a cone-shaped body" with a reversely turned flange 23, which acts like an elastic ring for supporting the main or central section of the diaphragm for vibratory movement with substantially uniform amplitude over its entire surface. The diaphragm 20 may consist of suitable sheet material, such as paper, celluloid, varnished fabric, sheet metal like brass or aluminum, and other materials suitable for acoustie reproduction. It will be noticed that the inverted flange 23 turns backward at a substantial angle. The practical effect ofthis construction is to place the conical flange 23 under tension when the edge ofthe diaphragm is clamped to ring l8. Expressing'this differently, I might say that the tensioned elastic flange 23 exerts an outward radial pull on the main body of the diaphragm,-whereby the latter is held constantly under tension. The flange 23 may therefore be -described as an annular tensioning support for -'the main body of the diaphragm, and it is preferably an integral resilient extension of the dia- 'phragm, although it can be made separate and of a. diiferent material. For instance, the body of the diaphragm can be made of heavy paper :or stiffened fabric, and the annular tensioning flange 23 may consist of thin sheet metal, which will hold the central non-metallic body constantly under tension in'all kinds of weather. The diaphragmlll'may' be lessthan nine inches across to give loudspeaker reproduction of adequate vol- --ume.' The arrangement of the coacting conical rings l3 and 2| makes it very easy to attach and Temove the diaphragm;

.if: .The casing l4 carries a field magnet structure indicated as a whole by M and comprising a EUJ-sliaped base frame 24- provided with lateral limbs 24', a central cylindrical-pole piece 25, and -a polar cross-arm 26 formed with a central ring '21. A screw 28 secures the central pole piece 25 :itothe U-shaped frame 24,.and the cross-arm :26 'is attached by screws 29 to the side limbs 24' of frame 24. The parts 24, 25 and 26 of the field magnetyare preferably made of metal having a hi h magnetic permeability, and the screws 28 and 29 may also be of magnetic material. The cross-arm 2,6 with its central ring 21 is best'made :asa single casting, and the U-shaped frame 24 may be solid or laminated. If solid, this frame maybe a casting, or bent into shape from a straight bar. If the frame 24 is built up of laminations, .as shown at 24a in Figs. 5 and 6, the .crossarm26 is attached to the laminated frame byproviding the ends of the arm with slots 26' vadapted-to receive the ends of the frame in a tight fit, and bolts or rivets 30 secure the parts together.

Two magnetizing coils 3| are mounted on the cross-arm 26 on opposite sides of the central ring 21. These coils are connected in a circuit of substantially constant current to produce a magv netic field in which the cylindricalprojection 25 constitutes one pole and the ring 21 the other pole, For convenience I have assumed in Figs. and5 that the polar projection 25 is a north .pole. andthe ring 21 a south pole, but the polariinto sound or other movements.

The vibratory coil 33 is mounted on a light cylindrical shell indicated as a whole by 34,, and best shown in the perspective view of Fig. 3. The shell 34 comprises a main section 35, an enlarged extension 36 and a conical flange 31. The cylindrical sections 35 and 36 are separated by an annular shoulder 38, and the'free end, of section 35 terminates in lugs 33. It is desirable to make the shell 34 as light as possible, and it should be non-magnetic. This shell may be spun of sheet brass or aluminum as a single member, or it may be shaped in'one piece from flber, paper pulp, stifi paper, celluloid, bakelite, and other non-magnetic materials suitable for the purpose. If the shell is'of metal, the section 35 is preferably slotted, as indicated. at 40, to cut down the disturbing effect of eddy currents." Thecoil 33 is mounted directly on section 35 of shell 34, and

is rigidly held between the shoulder 38 and lugs Normally, the lugs 39 are axial extensions of cylinder 35,.and they arebent' laterally against the coil 33 after the latter has been slipped in place. This makes it possible to use amachinewound coil. The coil-retaining means 3'9"need not be in'the'form of spaced lugs or projections, but may be a continuous lateral flange spun or turned over at the edge of shell 34 after the 0011 has been inserted. If desired, the coil 33- may be wound directly on shell 34 by mounting the latter on a mandrel in a winding machine. The conical flange or enlargement 31 serves as means for attaching the shell 34 to the conediaphragm 20. Rivets 4| maybe used tosecure'the flange 31 to the diaphragm, but it is'possible to connect these parts by cement or otherwise, depending on the material of the diaphragm. The shell 34 is thus rigidly attached to the center of diaphragm 20 and held in axial alignment with the central pole piece 25, so'that no portion of coil 33 and shell 34 shall strike the surrounding pole pieces. As seen in' Fig. 1, the outer surface of coil 33 is substantially flush with the enlarged cylindrical extension 36 of the supportingshell 34.

A compensating coil 42 may be mounted on the magnetic pole piece 25 to neutralize or oppose the variable magnetic field set up by the current impulses in-speech coil 33. Since the coils .33 and 42 are closely coupled magnetically by the pole piece 25, on which they are'axially arranged, it is evident that, by winding the coils in opposed relation and connecting them in series or parallel, the flux disturbances set up in themagnetic core 25 by the current impulses in coil 33 are neutralized by the opposing flux produced in core 25 by the current impulses in coil 42. While the vibratory coil 33 is long and thin so as to operate in a narrow airgap, the compensatingcoil 42 may be wound flat and thick to occupy minimum are: preferably molded from suitable insulating material, like bakelite} porcelain, fiber; glass and soon: Ea'ch insulating block "carries two pairs of' 'screw-threaded metal sleeves 46; which are electrically connected at one end by a-metal'plate 41; Into the metal sleeves"45'areinserted connecting screws WhiChjI have indicated consecu tively bythe r'eierence numerals 48 55.' The screws 48"55"-'act as 'binding posts for conductors marked 5663, respectively. .These circuit connections are best shown in Fig. '4',where it will beseen thatconductor 56 connects one end of the vibratory speech coil to bindingpost' 48, conductor- 5Tv connects one end of the compensatingcoil 42 to bindingpost 49, conductor 60 connectsone end ofmagnetizingcoils 3| to binding post 52', and

conductor 6| connects the'otherend of those coils to'binding .-post. 53. The 'co'ils 33 and'42 are connected in series by conductorlitand the magnetizing coils 3| are connected in series by conductor 65. If the compensating-coil is omitted, con-' ductor 51 goes to the other endof speech coil 33.

The-leads58 and 59 connect the'coils'33 and-42 the ba'ck cover plate 66 is removed from the cylindrical housing" 13. Screws 6'! secure the cover plate-66 in place and yet permit e'asyremoval F thereof. The cover-plate is prefer-ably of non-' metal like brass. aluminum and magnetic sheet the likeln g I The magnetic frame =M is rigidly held in cas ing I4 by aring 68, which is in adjustable screw-' threaded engagement with the inner wall of the casing, as indicated at 69. By simply tightening the ring 68; the magnetframe is rigidly clamped within" the casing |4=between-the ring andthe annular shoulder 13 of the casing. --'Tl'1ie'inner shoulder"!!! is' in effect a'continuation of the outer shoulder lfi'previously referredto. The ring 68 is provided with recesses H to permit thein'sertion of a suitablewoolforscrewingthe ring into scribed will be clearly-understood by those who are familiar with: the art of electric sound reproduction, and I need only say that, when current impulses pass through the speech coil33, the latter vibratesaxially in the annular'magnetic airgap32." The amplitude and direction of vibration depend onthe: strengthand direction of the current flowing through the coil at any in-'- stance: Since the coil 331s rigidly connected to the diaphragm 20, the vibratory movements of the coil'are accompanied by corresponding move-' ments of the diaphragm. As the magnetizing coils 3| are at right angles to the speech coil 33, the transformer or inductive effect between these coils is reduced to a minimum and can be practically eliminated in a properly designed instrument. The neutralizing action of coil 42 on core 25, as already explained in detaiLsupplements the neutralizing action of theright an gled "relation between coils 3| and33 to eliminate the'mag netic disturbances that would otherwise be produced by the variable currents in coil33. Conse'-;

quently, the resultant reproduction isnot only free from-distortion, butis of substantially -equal1v efiiciency over the whole audio-frequencyscaleJ In the modification of Fig. 9,' the magnetizing coils 3| are mounted on an arm'14, to whichlthe central pole piece 25 is 1 connected."The polar arm 26 is in thiscon'struction provided with' 'lateral extensions 15, which are connected to the ends of arm 24 by'b-olts or screws 16. 'By comparing Figs. 1 and 9, it will beseen that :thefield magnet structurelis practicallythesame, except that in the first casethe'frame 24 is U'-'shaped andin Fig. 9 the polar arm26 is .U -sha'ped. Con sidered mechanically, however; the two construetions are substantially alike. By :mountingithel magnetizing coils 3| in Fig. 9 on the base mem-,; ber 14, they are still at right angles to the speech. c0il-.33, and they have the additional-advantage of being farther removed from the speech coil, so that the variable magnetic flux {produced :by.

the speech coil cannot 'interlink-with.theiturns' of the magnetizing coils, at least not tofany: ap.-.

preciable extent. Since the mountingiof the field.

magnet in Fig; 9 is the same as in Figtl zitwill not be necessary to repeat a detailed description of thatmounting. It is sufficient;thatEPcorre-- sponding parts in Figs. 1 and 9areindicatedby like reference numerals.:;:. The same remarks apply to like parts in the circuit diagrams of Fig .14 and 10,. the latter relating to'Fig;.9. w; 1 H The cone diaphragm 11 of Fig. 9v has a central.

conical portion l8 arranged reversely with respect" to the main body of the diaphragm and preferably of difierent material.. The purpose of ,thecomr. pound diaphragm ll-18 is to make, it respondwith. equal efiiciency to high and low frequencies. Thus, the main body of the diaphragm" may be madev of stiff paper, or other material radapted torespond'best at low frequencies, whilethe pen-.- tral conicalportion I8 is made of thin sheet metal like brass, aluminum, .or other material adapted to operate most efficiently on the higher frequencie's.

surrounding body by rivets 19, or by cementing the two parts together. The conical section 'lfl has an outer flange adaptedftoalie fiat against. the main portion of the diaphragm to permit the The'central cone 18 covers aniopjenef ing 11?, of the diaphragm and-.isfsecured to .the I two partsto be securely connected" in any p'rac tical way. The centralcone "of the diaphragm carries a bolt or rod 8| to which acylindrical shell 82 is rigidly connected. The shell .82: ishsimilar;

to shell34 of Fig. 3, exceptthat; insteadeof 1a;

conical flange 31, ithas a head 83 whichis clamped;

between, nuts 84 on boltBl. Otherwisewhatwas: said about shell 34 applies to shell '82.

The speech coil supported on shell 82in may. be a single coil, but. in-certain constructions I prefer to make the speech coil of-two, windings,

marked'33a, and 33b in Figs. 1 9 and 10.;.:These. windings are connected in parallel between a pairofconductors 86 and in, series-,withthe: com.'-.

pensating coil 42, if the latter is used. T-hewinde,

high frequencies, respectively. In other words,

we have here a speech coil composed of sections which operate individually with higher efiiciency over certain bands of frequencies,so that the coil 

